1. Scope of Invention
This invention relates generally to large electrical power transformers, not to smaller distribution transformers, and more particularly to shell-form or pancake conductor coils having improved lateral and longitudinal turn strength.
2. Prior Art
The shell-form style of transformer construction is essential to current modes of electrical power generation and transmission. The inherent mechanical strength of the design to resist forces generated during external short circuits and other electrical surges is excellent. These transformers are called upon to either step up or step down voltage on a highly efficient level.
Such step up or step down transformers incorporate a number of individually wound shell-form or pancake coils which are connected "start-to-start" and "finish-to-finish" to form a "group" of high or low voltages. These groups of coils are placed in certain sequences with appropriate insulation barriers when designed as either single phase or three phase transformers. Because equal electrical current flows through each group of coils in the same direction, magnetic fields which are developed are canceled between each of the coils within the group, thus creating mechanical forces of attraction which drastically increase when large short-circuit current flows during electrical faults occurring external to the transformer.
As a result of these increased attractive forces, some of which are non-axial (radial) because of varying coil inside and outside dimensions to the electrical core steel, the edge margins of the coil can laterally or longitudinally deform, either momentarily or permanently, to the extent that a short may occur between spaced adjacent coils or between turns in the same coil.
Additionally, during coil manufacture, a single length of a conductor groups forming an electrical turn is spiral wound on a horizontal surface around a central mandrel having a shape of a core of the transformer into which the flat coil will be installed as part of a group. Multiple turns of the conductor groups are required in the spiral winding to complete the coil. The conductor groups of each turn are pre-wrapped with an insulating paper material which may include an epoxy thermo set adhesive that, when cured, bonds all conductor groups to add strength to the otherwise somewhat fragile coil as it is lifted and transferred into its assembled position after the coil forming process is complete.
In U.S. Pat. No. 4,489,298, Hall teaches the utilization of a dielectric cover for enclosing the end strands whereby the end strands of adjacent layers are sufficiently insulated to prevent electrical breakdown therebetween. However, this device is not subjected to high mechanical forces, nor does it increase the resistance to deformation during fault current situations.
Trunzo, in U.S. Pat. No. 3,934,332 teaches a method of making electrical coils having improved strength and oil permeability. It is here that an aspect of the teaching of the addition of an adhesive (epoxy) resin to the insulation wrapped around the elongated conductor to be spiral wound into a coil is taught. Another form of heat cureable adhesive addition for strength and insulation is taught by Palmer in U.S. Pat. No. 3,170,134. However, the epoxy resin is used for coil turn stability in these distribution transformers as a manufacturing aid rather than to control mechanical forces.
The following additional U.S. patents are shown to represent the current state of the art in distribution transformer coil construction, method of manufacture and/or utilization: